When Hosts Go Extinct, What Happens To Their Parasites?

Well, in many cases, of course, they go extinct themselves. But there is another possibility:

"There is a distinct possibility that declines in host species could drive parasite species to switch onto alternative hosts, which in turn could escalate the rate of emerging pathogens and parasites both for humans and our domesticated animals and plants," Dunn says. "Put simply, when a host becomes rare, its parasites and mutualists have two choices: jump ship to another host or go extinct. Either situation is a problem."

Dunn noted that the regions where new human diseases, such as bird flu, are emerging coincide with the regions where the most mammal and bird species are endangered. "We have long talked about the negative consequences of the endangerment of the species we love," he says, "but getting left with their parasites is a consequence no one bargained for."

Source. A sobering thought, no?